The invention concerns the pre-lubrication of the zinc coated surface of plates coated with zinc or zinc-based alloy and the treatment of these plates in aqueous solutions containing sulphates.
The document entitled xe2x80x9cThe influence of some sulphur-containing anions on the anodic behaviour of zinc in an alkaline mediumxe2x80x9d, written by S. S. Abd El Rehim et al., published in xe2x80x9cJournal of Electroanalytical Chemistryxe2x80x9d, 401 (1996) 113-118, describes, in an alkaline medium (|NaOH|=0.1 M, therefore pH=13), the oxidation of zinc into zinc hydroxide Zn(OH)4= ion in a first step, then the oxidation of this hydroxide ion into zinc oxide ZnO2 in a second step; this document also describes the influence of sulphur-containing species in solution, in particular of sulphate SO4= ions, on the second oxidation step.
According to this document, if the concentration of Zn(OH)4= ions attains the product of solubility, the zinc hydroxide Zn(OH)2 precipitates and forms a passivating film on the surface.
With reference to FIGS. 2 and 4, this document (cf. page 115) teaches that the presence of SO4= anions stimulates the dissolution of the zinc; this effect might come from the adsorption of these anions on the zinc-coated surface which facilitates oxidation of the zinc of this surface by these adsorbed anions.
With reference to FIGS. 2 and 5, this document (cf. page 115) also teaches that too high a concentration of anions in the alkaline solution provokes rupture of the passivation film formed after the first oxidation step.
The domain of concentration studied in this document covers the range 0.05 to 1.7 mole SO4= per liter.
Documents JP 61-60915, 63-46158, 63-46159 and EP 0 339 578 describe anodic treatments of zinc-coated steel plates in aqueous solutions containing sulphates and the use of these treatments to colour the zinc-coated surfaces of these plates.
The concentration of electrolyte in the treatment solution is included between 70 and 200 g/l; the concentration of sodium sulphate is for example 150 g/l, i.e. about 1 mole/liter.
Document JP 63-274797 describes the use of a treatment of the same type for improving the aptitude to phosphatisation of steel plates coated by electrodeposit of a zinc-based alloy containing nickel.
The treatment solution then contains, in addition to sulphates (for example: magnesium, sodium or aluminium sulphate at 150 g/l), carboxylic acids (for example citric, maleic, salicylic at 30 or 40 g/l).
The pH of the treatment solution is included between 4 and 5.5.
The anodic treatment is conducted under a current density included between 30 and 200 A/dm2 until the quantity of electricity consumed is included between 50 and 500 C/dm2 of surface to be treated.
According to this document, this treatment improves the aptitude to phosphatisation because it results in the elimination of the superficial traces of zinc hydroxides xe2x80x94Zn(OH)2xe2x80x94 and makes it possible to improve the surface reactivity.
The plates resulting from these treatments under anodic polarisation in aqueous solutions containing at least 0.05 mole of sulphate ions per liter, particularly more than 10 g/l of sulphate ions, pose problems in deep-drawing and forming.
Patent EP 0 489 105 describes a method of treating the metallic surface of a plate, particularly a steel plate, intended for preparing said plate for deep-drawing and/or for protecting it against corrosion, in which:
there is applied on this surface an aqueous solution of a water-soluble salt of an alkaline metal, particularly potassium phosphate,
said surface is dried,
then at least one operation of oiling of said surface is carried out.
The conditions of application and of drying are adapted in order to obtain a deposit of phosphate of surface density included between 5 to 40 mg/m2.
The oil used may be an oil for temporary protection against corrosion and/or a lubricating oil for forming, particularly deep-drawing.
Thus, before storage and/or transport, a first oiling for protection may be effected; after taking from stock, a second oiling for lubrication may be effected, for preparing forming, particularly by deep-drawing.
According to this document EP 0 489 105, the phosphatisation treatment, prior to oiling, makes it possible substantially to improve lubrication at the moment of forming: it is therefore question of a pre-lubrication treatment.
The zinc-coated plate obtained is provided with a pre-lubricating deposit based on phosphate.
The lubricating effect of this phosphatisation treatment sometimes proves to be insufficient on zinc-coated plates; furthermore, this treatment generates effluents containing phosphates, which it is desired to avoid.
The invention has for an object to provide a pre-lubricated zinc-coated plate with higher performances than those obtained by phosphatisation and to offer a treatment of pre-lubrication of zinc-coated plates more efficient than a treatment of phosphatisation and more respectful of the environment at the level of the effluents that it generates.
To that end, the invention has for its object a steel plate coated with a metal layer based on zinc, characterized in that:
said metal layer is itself coated with a layer based on zinc hydroxysulphate,
the surface density of sulphur corresponding to said layer of hydroxysulphate is more than 0.5 mg/m2.
Other advantageous characteristics of the plate according to the invention are indicated in the dependent Claims.
Several methods of hydroxysulphation make it possible to arrive at the pre-lubricated zinc-coated plate according to the invention.
The invention therefore also has for an object a first method of obtaining a plate according to the invention from a steel plate coated with a metal layer based on zinc comprising the steps consisting in applying on the zinc-coated surface of the starting plate an aqueous treatment solution containing more than 0.07 moles of sulphate ions per liter, in polarising in anodic manner said surface so as to cause a polarisation current to circulate, then rinsing said surface, then drying it, characterized in that
the pH of said solution is greater than or equal to 12 and less than 13,
the quantity of electric charges, circulating during the treatment through said surface and generating on said surface the deposit of a layer including sulphur, is adapted so that the quantity of sulphur obtained in said layer of hydroxysulphate exceeds 0.5 mg/m2.
Other advantageous characteristics of the first method of obtaining the plate according to the invention are indicated in the dependent Claims.
In practice, it is possible to determine, for predetermined conditions of use of the treatment solution, the minimum quantity of charges that should be circulated in order to obtain a deposit presenting a sulphur content of 0.5 mg/m2, to carry out this method, the quantity of charges used must in that case be greater than this minimum quantity.
The plate obtained by this method, then oiled, offers very good tribological properties well adapted to forming, particularly by deep-drawing: this hydroxysulphation treatment therefore has an effect of pre-lubrication.
By optimalizing the thickness of the layer of hydroxysulphate deposited, the pre-lubricating effect obtained is greater than that contributed by a treatment in a solution of phosphate as described in EP 0 489 105.
It is observed that this effect of pre-lubrication comes from the nature of the deposit: the infrared reflection spectrum of the deposit obtained is shown in FIG. 3 (reflectance in % as a function of the wave number in cmxe2x88x921); here it is essentially a question of a layer of zinc hydroxysulphate, also called basic zinc sulphate; this hydroxysulphate would correspond to the general formula:
[Znx(SO4)y(OH)z, tH2O],
where 2x=2y+z, with y and z different from zero; preferably, z is higher than or equal to 6, and more preferably z=6 and 3xe2x89xa6txe2x89xa65; according to the spectrum of FIG. 3, x=4, y=1, z=6 and t=5.
In order to be efficient at the level of pre-lubrication, this layer of zinc hydroxysulphate should adhere to the surface treated: the conditions relating to the pH of the treatment solution and the step of drying at the end of treatment are decisive to that end.
If the pH of the solution is less than 12, no hydroxysulphates adherent on the surface to be treated are formed; if the pH of the solution is greater than or equal to 13, the hydroxysulphate re-dissolves and/or decomposes into zinc hydroxides; in that case, we are again under conditions similar to those described in the document of S. S. Abd El Rehim cited hereinabove.
After rinsing but before drying, the layer deposited on the plate presents the appearance of a gel which is still only sparingly adherent; drying is adapted in order to eliminate the residual liquid water from the deposit and enables the layer to adhere better on the plate.
When sodium sulphate is used in the solution, if the concentration of sodium sulphate is less than 10 g/l in the solution, hardly any formation of layer of hydroxysulphate on the surface is observed; more generally, it is therefore important that the concentration of sulphate ions be greater than 0.07 moles per liter.
Preferably, the concentration of sulphate ions is less than or equal to 1 mole/liter; in the case of using sodium sulphate, at concentrations greater than 142 g/l (equivalent to 1 mole SO4=/liter), for example 180 g/l, a reduction is observed in the yield of formation of the layer of hydroxysulphate.
It was observed that the pre-lubricating effect of the treatment was obtained only if the thickness of the layer deposited corresponded to more than 0.5 mg/m2 in sulphur equivalent, preferably at least 3.5 mg/m2 in sulphur equivalent.
Inversely, it was observed that the pre-lubricating effect of the hydroxysulphate layer decreased if the quantity of sulphur deposited largely exceeded 30 mg/m2, apparently due to the degradation of the adherence of this layer; it was also observed that, if the quantity of sulphur deposited exceeded 27 mg/m2, the phosphatisability of the plate obtained degraded, the crystals of phosphate deposited in that case being too large.
Thus, in order to obtain a significant pre-lubricating effect, the quantity of hydroxysulphates deposited must be greater than 0.5 mg/m2 and less than or equal to 30 mg/m2 in sulphur equivalent, preferably included between 3.5 and 27 mg/m2 in sulphur equivalent.
The charge density applied must therefore be adapted to this quantity of hydroxysulphates adapted to procure this significant pre-lubricating effect.
Preferably, the charge density applied is thus preferably included between 10 and 100 C/dm2 of surface to be treated.
If the charge density exceeds 100 C/dm2, it is observed that the quantity of sulphur deposited on the surface no longer increases and even decreases.
This first method of obtaining a plate according to the invention therefore makes it possible to form on a zinc-coated surface a layer based on hydroxysulphate, which is both sufficiently thick and adherent.
Thanks to the anodic polarisation of the zinc-coated surface to be treated, the zinc dissolves rapidly in the immediate proximity of the zinc-coated surface, this promoting precipitation of zinc salts on this surface.
Thus, in order to effect this treatment as productively as possible with a satisfactory faradic output, the deposit of the hydroxysulphate layer should be effected under a current density of high polarisation, particularly higher than 20 A/dm2.
For a current density less than or equal to 20 A/dm2, the yield of deposit is very low and the sulphur content of the layer deposited does not enable the optimum pre-lubricating effect to be obtained.
Experimental tests have shown that, for a predetermined electric charge density, for example 20 C/dm2, the quantity of sulphur deposited on the surface to be treated was an increasing homogeneous function of the current density in the range of values included between 20 and 200 A/dm2; preferably, a current density which is as high as possible will therefore be chosen, for example 200 A/dm2.
As counter-electrode, a cathode made of titanium may be used.
The temperature of the treatment solution is generally included between 20xc2x0 C. and 60xc2x0 C.; preferably, one proceeds at a temperature higher than or equal to 40xc2x0 C., so as to increase the conductivity of the solution and to reduce the ohmic losses.
The speed of circulation of the solution on the surface of the plate does not, here, have any decisive influence on the treatment according to the invention.
After formation of the layer of hydroxysulphate on the surface, the treated surface is rinsed abundantly with demineralized water; within the framework of this first method, the step of rinsing is important to eliminate the alkaline reagents on the surface of the deposit, which would cause problems of corrosion.
The plate thus pre-lubricated by the treatment according to the invention presents a homogeneous coloration, a little stronger with respect to that of a non-treated zinc-coated plate; nevertheless, this treatment does not colour the plate, as in documents JP 61-60915, 63-46158, 63-46159 and EP 0 339 578 already cited; observed under a microscope, the deposit resulting from the treatment according to the invention is in the form of scattered plates; it has been noted that the density of plates increased with the quantity of sulphur deposited per surface unit.
In this first method of obtaining a pre-lubricated zinc-coated plate according to the invention, the zinc necessary for the formation of the pre-lubricating deposit of zinc hydroxysulphate comes from the anodic dissolution of the zinc under the effect of the polarisation of the zinc-coated surface.
This first method of obtaining obviously presents the economic disadvantage of requiring a polarisation installation.
In order to overcome this drawback, the invention also has for an object a second method of obtaining a plate according to the invention from a steel plate coated with a metal layer based on zinc comprising:
a step of application, on the zinc-coated surface of the starting plate, of an aqueous treatment solution containing more than 0.01 mole of sulphate SO4= ions per liter,
and a subsequent step of drying,
characterized in that:
said treatment solution contains Zn2+ ions at a concentration greater than 0.01 mole/liter,
the conditions of application, in particular the duration, the temperature of said solution, the concentration of SO4= ions and of Zn2+ ions in said solution, are adapted so that the quantity of sulphur obtained in said layer of hydroxysulphate exceeds 0.5 mg/m2.
Other advantageous characteristics of the second method of obtaining the plate according to the invention are indicated in the dependent Claims.
This second method of obtaining a plate according to the invention does not require a polarisation installation.
For example, the treatment solutions are prepared by dissolution of zinc sulphate in pure water; for example heptahydrated zinc sulphate (ZnSO4, 7 H2O) is used; the concentration of Zn2+ ions is in that case equal to that of the SO4=anions.
The pH of the treatment solution used for this second method is generally much less basic than that of the treatment solution used for the first method; the pH of the treatment solution preferably corresponds to the natural pH of the solution, without addition of base nor of acid; the value of this pH is generally included between 5 and 7.
The treatment solution is applied on a zinc-coated surface of plate in conventional manner, for example by immersion, by spraying or by coating.
The conditions of application, such as the duration for immersion and spraying or the quantity for coating, such as the temperature of solution, such as the concentrations of SO4= and Zn2+ ions, are adapted in manner known per se for the quantity of sulphur obtained in the final layer of hydroxysulphate to exceed 0.5 mg/m2.
It has been observed that, if the concentration of SO4= ions and/or if the concentration of Zn2+ ions were less than 0.01 mole per liter, such a layer of hydroxysulphate would not be able to be formed.
The treatment solutions used preferably contain between 20 and 160 g/l of heptahydrated zinc sulphate, corresponding to a molar concentration of Zn2+ or SO4=ions included between 0.07 and 0.55 mole/liter; in this range of concentrations, it has been observed that the speed of deposit was little influenced by the value of the concentration.
After application and before drying, the layer deposited on the plate is adherent; drying is adapted to eliminate the residual liquid water from the deposit.
Between the step of application and the step of drying, the plate is preferably rinsed so as to eliminate the soluble part of the deposit obtained; the absence of rinsing and the obtaining of a deposit partially dissolvable in water which results are not very detrimental to the pre-lubricating effect, as long as the deposit obtained indeed comprises the pre-lubricating layer of hydroxysulphate insoluble in water on contact with the plate.
The plate obtained by this second method presents intrinsic and extrinsic characteristics comparable to those of the plate obtained by the first method; the infrared reflexion spectrum of the deposit of hydroxysulphate under glancing incidence is given in FIG. 4 (reflectance in % as a function of the wave number in cmxe2x88x921); here it is also essentially a question of a layer of zinc hydroxysulphate which would correspond to the general formula:
[Znx(SO4)y(OH)z, tH2O],
where 2x=2y+z, with y and z different from zero; z is preferably higher than or equal to 6; and more preferably z=6 and 3xe2x89xa6txe2x89xa65, according to the spectrum of FIG. 4, x=4, y=1, z=6 and t=3; the deposit of hydroxysulphate obtained is finely crystallized and highly covering.
This second method presents the following advantages over the first method:
the anodic polarisation of the zinc-coated surface is not necessary in order to obtain the desired pre-lubricating effect,
the deposit of hydroxysulphate obtained is more homogeneous.
Numerous parameters may have a significant influence on the speed and/or the thickness of the deposit of hydroxysulphate obtained:
the conditions of applications of the solution:
the duration of application: the deposit obtained after 300 seconds may present a surface density double that obtained after 100 seconds;
the renewal of the solution in the vicinity of the zinc-coated surface: in the case of application in immersion, appropriate stirring of the bath makes it possible to double the speed of deposit;
the temperature of the treatment solution: the range of optimal temperatures is determined, for example between 40xc2x0 C. and 60xc2x0 C.
Concerning the incidence of the concentrations of SO4= ions and Zn2+ ions in the treatment solution, it is observed that there exist thresholds of concentration below which no pre-lubricating deposit is obtained, but it is also observed that too high concentrations do not substantially improve the speed of deposit and can even reduce it slightly.
In order to carry out this second method, these parameters are optimalized in manner known per se in order to obtain a deposit of hydroxysulphate according to the invention, i.e. containing a quantity of sulphur greater than 0.5 mg/m2.
According to a variant of this second method, the treatment solution contains an agent oxidising zinc, such as hydrogen peroxide; this oxidising agent may have a very marked hydroxysulphation accelerator effect at low concentration; it has been observed that the addition of only 0.03%, viz 8 10xe2x88x923 mole/liter of hydrogen peroxide, or of 2 10xe2x88x924 mole/liter of potassium permanganate in the solution made it possible to double (approximately) the speed of deposit; on the contrary, it has been observed that concentrations 100 times greater no longer made it possible to obtain this improvement of the speed of deposit.
Taking into account the pre-lubricating effect of the deposit of hydroxysulphate, the invention finally has for an object a method for forming a steel plate coated with a metal layer based on zinc comprising the steps consisting in treating the surface of said coated plate according to the first or the second method described hereinabove, in applying a film of lubricating oil on said dried treated surface and in forming said plate proper.
These methods are applied for example on plates zinc-coated by electrolysis; for plates galvanized by immersion, the second method of treatment will preferably be used; for plates coated with zinc alloy, the second method of treatment will for example be used without rinsing.